Amphionides reynaudii is a species of caridean shrimp, whose identity and position in the crustacean system remained enigmatic for a long time. It is a small (less than one inch long) planktonic crustacean found throughout the world's tropical oceans, which until 2015 was considered the sole representative of the order Amphionidacea, due to unusual morphological features. Molecular data however confirm it as a member of the caridean family Pandalidae, and the confusion of morphology is because only larval phases have so far been studied.
Amphionides specimens observed have been up to 25 mm (1.0 in) long. In view of adult shrimp morphology, Amphionides appears unusual, with many body parts being reduced or absent. For example, it has only one pair of mouthparts – the maxillae – the mandibles and maxillules being vestigial.
Males and females differ in the form of the antennae, and also by the presence in males of the eighth thoracic appendage, albeit in a reduced form. This is the site of the male gonopore (the female's gonopore is on the sixth thoracic appendage). The first pleopod of the female is greatly enlarged and almost encloses the enlarged carapace. This is assumed to be a chamber in which the eggs are fertilised and retained until hatching. The more streamlined carapace and pleopods of the male make it more hydrodynamic, so fewer males are caught than females.
A. reynaudii has a cosmopolitan distribution in the world's tropical oceans. It is planktonic, inhabiting waters less than 100 m (330 ft) deep as a larva, but more often at depths of 700–1,700 m (2,300–5,600 ft) as an adult.
